1. Field of the Invention
The present invention relates to an automotive steering system.
2. Related Art
In conventional steering systems, the steering movements initiated by the driver via the steering wheel are transferred to the steered wheels by means of a steering gear, and as the case may be, aided by an assist force. Moreover, there are steering systems where the steering movements initiated by the driver are superimposed upon further steering movements initiated by an actuator. Such a system is known from DE-OS 40 31 316 (corresponding to U.S. Pat. No. 5,205,371). In this system, an actuator is provided between the steering wheel actuated by the driver and the steering gear, with the aid of which actuator an actuator movement can be superimposed on the driver's steering movements. Such a system assures that the steering angle of the steered wheels of a vehicle is not at a fixed ratio to the turning angle of the steering wheel actuated by the driver.
This makes possible, among others, a steering assist for the driver that is dependent on the steering angle and in addition, there are many options of variation given for having additional steering signals act upon the steering system. Envisaged with the additional steering signals, specifically, are increased road safety and/or travel comfort.
A problem that may be involved with such power-assisted steering that employs superimposition of movement is that the selection of a constant gear ratio for the steering gear as established in its design and that the actuator fails to achieve acceptable steering wheel moments or a good steering feel for all states of travel. In particular, the steering wheel moment must at large steering angles not become excessive while the vehicle is at rest, whereas with the vehicle traveling at high speed the steering wheel moment must not become insufficient.
An overlay steering mechanism of other design can be seen from DE-OS 42 43 267.
The objective underlying the present invention is to improve the driver's steering feel in case of a steering that employs superimposition of movement.